Protective device



Sept. 25, 1923. 1,469,127

E. B. WEDMORE PROTECTIVE-DEVICE Fi'led Dec. 10, 1919 Inventor: Edmund'BW dmore,

His Attorney,

Patented Sept. 25, 1923.

UNITED STATES EDMUND BASIL WEDMORE, 0F comm-vs ELECTRIC COMPANY, A coaroasrzon nn PATENT OFFICE.

nssienon T0 GENE YORK.

PROTECTIVE DEVICE.

Application filed December 10,1918. Serial No. 343,805.

To all whom it may concern:

Be it known that I, EDMUND B. WEDMORE, a subject of the King of Great Britain, residing at l Sneath Avenue, G'olders Green N. W. 4, England, have invented'certain new and useful Improvements in Protective Devices, of which the following is a specification.

My invention relates to protective sys tems and particularly to systems of the pilot wire type, and has for its object to extend the range of such systems so as to enable the protective apparatus to take care of faults between phases as well as faults to earth in a more effective manner than has hitherto been possible.

It is desirable to design such protective apparatus to give a low setting on faults to earth, but the relays with such a setting must not be affected by conditions which arise when the system is subjected to a severe short circuit between phases. The heavy currents flowing under such a condition cause a rise of potential on the pilot wires, resulting in a flow of capacity current. Current may also flow through the pilot wires due to lack of balance between the voltages set up in the protective transformers. These conditions may result in the protective apparatus operating and the switches being opened on sound feeders as well as on a faulty one.

The relays must'not be set to operate'except at current values above the limits reached by these capacity currents and unbalanced currents respectively. In certain designs for example, it has been found that if the apparatus is adjusted to withstand say 6000 amperes on a work-shop test, it is necessary, in practice, to employ relay settings as high as 200 to 400 amperes. The object of my invention is to extend this range, thus enabling lower settings to be employed or higher short circuit currents to be with stood, as may be required.

It is particularly important to obtain low settings on faults to earth, but on faults between phases, relatively higher settings may be employed with satisfaction, and my in vention consists in the employment of a combination of apparatus giving low settings on faults to earth, and able to withstand very heavy short circuit currents.

, The short circuit currents flowing on faults to earth are sometimes limited by resistance in the earth connection, and do not reach the same maximum value as is reached on short circuits between phases According to my invention, I employ a combination of apparatus which is r latively sensitive I: "faults to earth, and able to withstand the relatively low maximum currents flowing on these f nits, while it is relatively insensitive on faults between phases, and able to wvithstand very heavy currents flowing under these conditions.

As hitherto constructed the protective transformers have commonly been designed with an air gap in the magnetic circuit, and such transformers are influenced by their surroundings, which may affect the permeability of the air gap. Such transformers are also liable to be affected by stray mag-, netic fields from adjacent conductors. It is a further object of my invention to dispose of this difficulty and this is done by so disposing the parts that the air gaps are shieldtive system embodyii'ig my invention, and

Fig. 2 is a diagrammatic representation of a modification of my protective transformer.

Referring to Fig. 1, lines 1, 2 and 8 re resent the conductors of a th ee-phase distribution system, parts oi b ree sections of the line being shown. At the ends of the middle section of the line there are shown cooperating transfori'ners A and B connected in opposition, each of which comprises a core 4i prefenabl in the form of a ring or cylinder surroundin he conductors l, .2 and 3. This core is shown continuous but it may have air gaps symmetrically disposed with reference to the conductors. The core 4 is provided with either a continuous winding or a winding in the form of three separate coils 5 sysmmetrically disposed with relation t the conductors 1, 2 and 3. The outer core 4; corresponds to a wellknown design of core balancing transformer, which is sensitive to faults to earth and may be readily the apparatus will produce equal electromotive forces in the windings on the inner cores 6 of transformers A and B, and since these windings are connected in opposition, no operating current will flow in relays 8.

Consequently relays 8 will remain inoperative and the circuit in the sound section is not interrupted. I

In the construction described, the outer ring portion serves as a magnetic shield to the airgaps between the outer and inner cores. The arrangement described is one convenient for production, but I do not confine myself to this form. Fig. 2 illustrates amodification of my protective transformer in which the second set of windings is carried on polar-projections 12 on the outer core, projecting between the current carrying conductors 1, 2 and 3, and having an air gap at the centre.

The apparatus above described may be conveniently mounted in a cable box of the type usually employed for supporting the terminals of a 3-phase transformer.

An alternating current system may be provided with as many pairs of such transformers as there are feeders and apparatus for which it may be desired to provide a protective device. The feeder, section, or apparatus where the fault occurs, will be open circuited without disturbing other portions of the system which are sound. The

word section in the claims is intended to cover a feeder, transformer or any other portion of a system where protection is applicable.

l/V hat I claim as new and desire to secure by Letters Patent of the United States, is,-

1. In protective devices for alternating electric current systems in which the input of electric energy into a section of the system is normaly balanced against the output from said section, the'combination of balancing devices which are relatively sensitive on faults to earth only, and balancing devices which are insensitive on faults to earth and relatively insensitive on faults between phases.

2. An alternating electric current system provided with a protective transformer at two points of the system, windings on said transformers interconnected to balance the input of electrical energy into the section between said points against the output from said section, certain windings on each of said transformers being interconnected to produce a resultant electromotive force upon faults to earth in the section and certain windings on each of said transformers being interconnected to produce a resultant electromot-ive force upon faults between phases, said transformers being constructed so that the windings responsive to faults between phases are relatively insensitive while the windings responsive to faults to earth are relatively sensitive.

3. In combination with a three-phaseelectric' system, protective transformers at two points of said system, said transformers comprising inner and outer cores, air gaps between said cores, conductors between said cores in series relation with said system, windings on said cores interconnected to balance the input of electrical energy into the section between said points against the output therefrom, the .windings on said outer cores being adapted to yield operating currents on faults to earth in said section and the windings on said inner cores to yield operating currents on faults between phases in said section and said outer core forming a shield for said air gaps.

4. The combination of a three-phase current system, a protective transformer comprising a three-legged inner core provided with windings, an outer core provided with windings air gaps for said inner core, and conductors between, said cores in series relation with the conductors of said system, said outer core acting as a shield for the air gaps of said inner core.

5. In combination with an alternating electric current system, protective transformers at two points of the system, said transformers comprising inner and outer cores, windings on said cores, conductors between said cores in series relation withsaid system, said windings on the outer cores of said transformers being interconnected for opposing electromotive forces and relatively sensitive on faults to earth and said Windings on the inner cores of said transformers being interconnected for opposing electromotive forces and relatively insensitive on faults, between phases.

6. A protective transformer for alternating current electric systems, comprising an outer core provided with secondary windings connected in series, a three-legged inner core constructed to provide a high reluctance in the path of each leg, secondary windings on a plurality of the legs of said inner core connected in series, and a primary winding comprising conductors placed between each of the legs'of said inner core.

In witness whereof, I have hereunto set my hand this 24th day of November. 1919.

EDMUND BASIL WEDMORE. 

